1. Field of the Invention
The present invention relates to a power amplifying device.
2. Description of the Related Art
An amplifier used for amplifying electrical power supplies a high electrical power to a load at a relatively low output impedance. Hence, it is necessary to ensure that the impedance of the load to which the amplifier is connected is within a specified range. If the load is short-circuited, the impedance of the load is reduced to zero, and a large current flows into a power amplifying element during an amplifier output stage, leading to a failure of the power amplifying element used in the amplifier output stage. For this reason, when an amplifier is used to amplify electrical power, there is a required to always check the condition of the load connected to the amplifier, and to immediately interrupt the amplifying operation of the amplifier to protect an amplifier output stage circuit, if the impedance value of the load deviates from a predetermined range.
A conventional amplifier for amplifying electric power detects a load condition as described below. First, a detection resistor having a small resistance value is connected in series with the load to measure a voltage drop observed on the detection resistor due to load current. Thus, changes in the load current are detected so as to detect load conditions from the changes.
In other words, the load conditions cannot be detected unless the load current passes through the detection resistor. More specifically, an abnormal load condition cannot be detected unless an amplifying operation is actually performed under an abnormal load condition. Furthermore, since the detection resistor is connected in series with a load, a power loss inevitably results by the load current passing through the detection resistor. In addition, a large resistance element must be used as the detection resistor because of a heat generation problem caused by such power loss. This adversely affects efforts to provide smaller amplifiers.
Recently, in response to the demands for reduced sizes and higher efficiency of amplifiers, amplifiers using the xe2x80x9cclass D amplifying methodxe2x80x9d are becoming more popular.
According to the class D amplifying method, a input analog signal is first subjected to processing for modulation, such as pulse width modulation (PWM) or the pulse density modulation (PDM). Then, a signal that has been converted into a digital signal by the modulation is amplified. Thereafter, the amplified signal is passed through a low-pass filter to convert it back into an analog signal. A class D amplifier is able to amplify an analog signal simply by amplifying a digital signal (signal ON/OFF processing). Theoretically, therefore, 100% power efficiency can be obtained. This makes it possible to reduce the size of the amplifier by taking full advantage of the high efficiency.
Thus, the power loss attributable to the detection resistor mentioned above and the need for using large components because of the heat generation problem conflict with the advantages obtained by class D amplifiers. There is also a problem with the conventional method for detecting a load condition since an abnormal load current cannot be detected unless the load actually develops an anomaly.
A class D amplifier has a low-pass filter in its output section. When the amplifier is not under a load, a signal current will only pass the low-pass filter of the output section. In general, a filter of the output section is constituted by an LC low-pass filter, and therefore has a predetermined resonance frequency. For this reason, if a signal input to the class D amplifier includes such a resonance frequency, a resonance phenomenon occurs when the amplifier is not under a load. This causes a significant drop in the impedance of the output section of the class D amplifier, allowing a large current to flow, which may possibly lead to a failure of the class D amplifier. There has been a problem with the conventional method for detecting a load condition by using a detection resistor in that it is difficult to detect a no-load condition of the amplifier.
Accordingly, the present invention was developed with a view toward solving the problems described above. It is an object of the present invention to provide a power amplifying device that permits an abnormal load condition to be quickly detected so as to protect an amplifier output, and minimize power loss.
To this end, according to one aspect of the present invention, there is provided a power amplifying device which includes a power amplifying circuit and to which a load having a predetermined impedance is connected, including a test signal generating part that generates a test signal having a predetermined frequency, a signal level measuring part that measures a signal level of the test signal that has passed through a filtering circuit provided in an output part of the electric power amplifying device, a signal level comparing part that compares a measurement result supplied by the signal level measuring part with a predetermined reference signal level, and a control part that detects a condition of the load connected to the power amplifying device on the basis of a comparison result supplied by the signal level comparing part, and provides protective measures for the power amplifying device according to a result of the detection.